The invention relates to the manufacturing of rotors for electrical generators and more particularly to a method and apparatus for cutting out the slots for receiving the rotor windings.
The procedure for cutting the rotor winding slots out of the rotor is that for each winding groove a slot is first cut having parallel, possibly stepped walls out of the rotor block by means of a side mill cutter. The final groove form is subsequently worked out by finishing the slot walls. If the groove walls are also to be parallel in their final form, this finishing is limited to the working out of so-called locking grooves; however, a considerably more extensive finishing is necessary with modern generator rotors because the grooves typically have "conical" walls, i.e., walls which approach each other are used, and in many cases even grooves having conical and undercut walls are provided.
The finishing is actually made in different ways depending on the form of the grooves. Purely conical grooves are finished with a conical side mill cutter, whereas undercuts, including the locking groove, are cut by means of shank-type milling cutters. This procedure has many essential disadvantages. The work with shank-type milling cutters is quite time consuming because such cutters allow only a relatively low cutting speed and because the cutter is able to work at only a relatively low advance speed since it can only have a small shank diameter at a relatively great length. Because of this length to diameter requirement, it is very unstable in spite of having a rigid support. It is true that this disadvantage does not apply to conical side mill cutters, but in the case of such cutters the dressing of a worn tool is extremely time consuming.
To overcome these disadvantages it has already been proposed to treat winding slots of generator rotors by planing. This technique, however, could not be introduced in actual practice. The planing technique generally used is not applicable to the treatment of winding slots of generator rotors because modern generator rotors weigh several hundred tons. If such heavy workpieces are moved at the relatively high speed necessary for planing, considerable distances and periods of time and forces are required to slow down the workpiece. At normal planing, only one cutting edge is engaged at a time and many steps are necessary until the slot is worked out so that the problem of slowing down becomes even more important. In the case of conical slots, it is additionally required that the conical slot wall and the conical part of the slot wall, respectively, must be free from steps. This can be achieved only with very careful control of the planing machine. As the treatment of the winding slot of a generator rotor would take several weeks, an operating error or even a minor inadvertence may possibly destroy the work of several weeks. Even if the probability of an operating error or an inadvertence is quite minor, the damage which is likely to be caused is so important that in a practical sense the planing of winding slots of generator rotors could not be introduced.